This invention relates to a control device, or a controller, for controlling a physical condition such as temperature and pressure of a controlled system, a temperature controller for controlling the temperature of a controlled system, and a heat processor using such a temperature controller. More in detail, this invention relates to such a controller, a temperature controller and a heat processor carrying out a control by using a model such as the Smith method (also known as the Smith-predictor and the Smith compensated method).
It has been known in the PID control, say, of temperature to set the PID gain weakly in order to control the overshoot at the time of set point response. If the PID gain is made too weak for the control, however, it comes to take too long to reach the set temperature and hence such a control cannot be practically usable where a quick rise in temperature is required. In such a situation, therefore, the Smith method is sometimes used in order to control the overshoot at the time of set point response by compensating for the dead time while making it possible to raise the temperature quickly with a strong PID gain. The Smith method may be characterized as controlling a system by treating it as a controlled system without the dead time, carrying out a dead-time compensated control by internally setting a controlled model.
Since the Smith method can control the overshoot at the time of set point response, the PID gain can be set more strongly than in an ordinary PID control. For this reason, however, the problem of hunting becomes more troublesome than in an ordinary PID control in the presence of external disturbance. Another problem with the Smith method is that a controlled model must be set internally but that it is not easy to obtain parameters such as the process gain and the time constant that are necessary in setting such a controlled model.
It is therefore an object of this invention to control the problems of overshoot and hunting both at the time of set point response and disturbance response in a control by using a model and also to make it easier to obtain parameters for the model.
In view of the above, a controller according to this invention may be characterized as comprising means (xe2x80x9ca variable calculatorxe2x80x9d herein) for calculating and outputting a manipulated variable on a controlled system based on of a target value which has been set and a feedback value obtained by a measurement, a dead-time compensator for providing a dead-time compensated output using models based on the manipulated variable outputted from the variable calculator, and a switch which is for selecting between a dead-time compensated control by providing a dead-time compensated output to the variable calculator and an ordinary control providing no dead-time compensated output to the variable calculator and functions to switch to the dead-time compensated control at least during set point response of the controller. In the above, the ordinary control means a control not carrying out dead-time compensation, using no dead-time compensated output.
According to this invention, overshoot and hunting can be controlled because dead-time compensated control is carried out at least at the time of set point response. Since an ordinary control is effected at other times, hunting due to disturbances can be controlled better than if the dead-time compensated were continued.
According to one example of the invention, the dead-time compensator has two models, one simulating the controlled system having a first-order delay and a dead time (the xe2x80x9csimulating modelxe2x80x9d herein), and an idealized dead-time compensated model which simulates the controlled system having no dead time. The dead-time compensated control uses the Smith method with a stronger control gain than a control gain for an ordinary control. Thus, the target temperature can be obtained quickly while the overshoot is controlled because the Smith method is used with a stronger control gain during set point response of the controller. Since an ordinary control with a weaker control gain is effected at other times, the hunting caused by external disturbances can also be controlled.
A heat controller according to this invention may be characterized similarly as the controller described above except that the controlled physical variable is the temperature of the controlled system and have similar effects.
The variable calculator described above may be adapted to output PI or PID variables. Control parameters for the models may be determined by the step response method or the limit cycle method. They may also be determined by a manipulated variable and a detected temperature after the detected temperature has settled or from a change in the measured temperature after the control is temporarily interrupted. The parameters may include the process gain or the time constant. These parameters may be obtained first by obtaining the maximum slope and dead time as done conventionally and either the process gain or the time constant is obtained such that all parameters necessary for determining the models may be obtained.
Heat processors of this invention are characterized as comprising a controlled system, an apparatus for heating or cooling the controlled system, and a temperature controller as described above. Heat processors of this invention include apparatus for thermal oxidization used in the production process for semiconductors, diffusion furnaces, CVD devices and molding apparatus. The merits of the heat processors of this invention are the same the same as those of the thermal controllers of this invention.